A variety of different industrial applications require the insertion of various types of metal fasteners into thermoplastics components with high pull-out resistance, rotational resistance and resistance to vibration and back-out torque. In addition, numerous applications are encountered which additionally require a relatively high degree of uniformity of such fastener insertion wherein metal fittings such as bushes, studs and the like are required to be fully installed in precisely flush relation to an upper surface, e.g., of a thermoplastics component.
The known apparatus for installing metal inserts into thermoplastics material wherein the inserts are heated to cause localized melting and flow of the thermoplastics material to enable insertion to take place are subject to a number of drawbacks, particularly with hand operated insert installation apparatus. For example, certain known apparatus incorporate a head or driver which is reciprocated under manual control toward and away from a thermoplastics component and utilizes both heat, which is imparted to the insert through a mandrel or tip assembly of the tool, and mechanical pressure which is manually applied by the tool operator.
Examples of these known techniques are found in United Kingdom patent specification No. 1,415,513 entitled "Improvements in and Relating to the Application of Metal Fittings to Thermoplastics Components" published Nov. 26, 1975 and issued to Armstrong Patents Co. Limited and which discloses a method of applying a metal fitting and applying heat directly to the fitting by means of a heated tool to effect local softening of thermoplastics material around a pilot hole and then exerting a mechanical pressure by means of the tool on the fitting to penetrate into the component. U.S. Pat. No. 3,926,356 entitled "Inserts" issued Dec. 16, 1975 in the name of L. E. Still discloses an apparatus utilizing a double acting piston for sequentially driving inserts into a thermoplastics workpiece incorporating a shoe for preheating the inserts with the shoe being coupled with the mandrel reciprocated by the double acting piston for supplying inserts one at a time.
As is known, the thermoplastics component is normally temporarily located in a fixed jig or other suitable device. In most such devices, the depth to which the insert is installed is dependent upon operator judgment with varying results in consistency and uniformity of the insert installation. Other devices depend upon sensitive operator judgment as to both the pressure and the timing of the mechanical pressure or driving force to be applied to the insert relative to the condition of the surrounding heated thermoplastics workpiece. Premature application of the insert installation force may result in splitting or cracking, e.g., of a workpiece or undesired introduction of stresses which may later result in a damaged product.
These problems are particularly evident in applications involving workpieces having relatively thin sidewalls surrounding a preformed opening for receiving an insert. Such workpieces are particularly susceptible to an operator imparting an excessive insertion pressure before the workpiece has reached a critical temperature to permit full flow of its thermoplastics material ensuring solid bonding of that material with the insert without creating undesired and unnecessary stresses in the workpiece and concommitant damage later such as splitting of the workpiece shell.
One approach which has been utilized to overcome the above problems is a mechanical spring for driving a heated tool bit independently of its carrier once the carrier is moved to an actuating position. The spring is manually overridden upon movement of the driver into the actuating position after arrest of the bit in an insert engagement position to apply an insert driving force on the bit under the influence of the spring to fully install the insert into the workpiece upon subsequent softening of its thermoplastics material.